The present invention relates to the general art of wireless communications and particularly radio navigation and radio direction finding.
Time of Arrival (TOA) is a well known location determination method in which a receiver calculates its distance to a transmitting beacon based on the time it takes a signal to travel between the transmitter and the receiver, and multiplying this time interval by the signal propagation speed, typically the speed of light. Theoretically, calculating distances to three transmitters, and knowing these transmitters spatial coordinates, enables a receiver to calculate its own spatial coordinates, by resolving three quadratic equations based on the three dimensional Pythagorean theorem. Hence, this method is also known as trilateration, and is the basic algorithm employed by Global Navigation Satellite Systems (GNSS) such as the US GPS, Galileo and Glonass. The geometric representation of this method is of three spheres, each sphere having a transmitting satellite at its origin and a radius of the relevant TOA multiplied by the speed of light, these spheres intersecting and having two common points, wherein the receiver is positioned on one of these points. FIG. 1 illustrates that method in two dimensions. Likewise, three receivers linked together (and time synchronized) can determine the location of a compatible transmitter. The latter method is typically employed by terrestrial cellular systems, wherein a transmitting mobile device is located by a network of several receiving base stations.
Typically, the location determination infrastructure, such as GPS transmitters or cellular base stations, is time synchronized, however not with the terminals—GPS receivers or cellular devices. So since in many cases there is an unknown difference or drift between the time reference (clock) used by the receiver(s) and the transmitter(s), four reference points are required to fix a location, defining four equations accounting for three unknown spatial coordinates plus said unknown time difference. This variation of TOA is then called TDOA—Time Difference of Arrival.
In two dimensions, two receivers measuring the TOA of a signal broadcast by a transmitter, define two intersecting circles, having two common points, said transmitter located on one of these points.
Typically, TOA is calculated by subtracting from the measured receiving time instant, the detected transmitting time instant reported by the transmitter (by data carried on the transmitted signal), and multiplying the TOA by the speed of light (C) provides the range or distance between the transmitter and the receiver. However, if these receivers are synchronized in time with each other, but not with the transmitter, and if the discrepancy between the transmitter clock and the receiver clock is unknown, then this TOA×C provides a pseudo-range or pseudo-distance, which should be corrected accounting for the different time reference employed in the TOA measurement. Still, if two measured TOAs are subtracted from each other, the result is TDOA, which is independent of said clock difference. This TDOA does not represent a circle any more (in 2D, or sphere in 3D), but a hyperbola, since a hyperbola is the locus of points where the (absolute value of the) difference of the distances to two foci (the receivers positions, in our case) is constant. Therefore, the TDOA of a signal transmitted from a specific location and measured by two receivers, defines a Line of Position (LOP) in shape of a hyperbola (with two branches). In order to fix a position, up to an ambiguity of two points, two such LOPs are required, i.e. two TDOA measurements made by two pairs of receivers, possibly selected from three receivers. The 2-D hyperbolic navigation method is illustrated in FIG. 2.
Hyperbolic navigation systems based on terrestrial infrastructure of transmitters were popular during World War II, for fixing the position of a receiver onboard aircraft and ships. Some known hyperbolic navigation systems were: Decca (by the British Navy), Gee (by the Royal Air Force—RAF), LORAN and OMEGA (by the US Navy). Some of these systems employed measurement of time difference, while others were based on phase difference, yet the basic principle was the same. Anyway, these hyperbolic systems became obsolete when the GPS became operational, providing better coverage, availability and accuracy.
In the three dimensional space, two receivers still provide a single TDOA measurement, but the locus of points where the difference of the distances to the two receivers is constant is not a line—hyperbola, as in the two dimensional space, but a plane—hyperboloid.
So if a pulse is emitted from a platform, it will arrive at slightly different times at two spatially separated receiver sites, the TDOA being due to the different distances of each receiver from the platform. Given two receiver locations and a known TDOA, the locus of possible emitter locations is a one half of a two-sheeted hyperboloid, as illustrated in FIG. 3.
So, with two receivers at known locations, an emitter can be located onto a hyperboloid, and the receivers do not need to know the absolute time at which the pulse was transmitted—only the time difference is needed. Then, with a third receiver at a third location, a second TDOA measurement can be acquired and hence locate the emitter on a second hyperboloid. The intersection of these two hyperboloids describes a curve on which the emitter lies. If a fourth receiver is introduced, a third TDOA measurement is available and the intersection of the resulting third hyperboloid with the curve already found with the other three receivers defines a unique point in space. The emitter's location is therefore fully determined in 3D.
In practice, errors in the measurement of the time of arrival of pulses mean that enhanced accuracy can be obtained with more than four receivers. In general, N receivers provide N−1 hyperboloids. When there are N>4 receivers, the N−1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. In this case, the location problem can be posed as an optimization problem and solved using, for example, a least squares method or Kalman filter. Additionally, the TDOA of multiple transmitted pulses from a static or quasi-static emitter can be averaged to improve accuracy.
The following two papers provide a mathematical methodic explanation for the 3-D hyperbolic positioning based on TDOA:
Exact Solution of a Three Dimensional Hyperbolic Positioning System
By Ryan Stansifer
Department of Computer Sciences, Florida Institute of Technology, Melbourne, Fla. USA 32901 20 Sep. 2011 http://cs.fit.edu/˜ryan/cse4051/projects/multilateration/mult.pdf
Exact Solution for Three Dimensional Hyperbolic Positioning Algorithm and Synthesizable VHDL Model for Hardware Implementation
By Ralph Bucher
New Jersey Center for Wireless and Telecommunication, Department of Electrical and Computer Engineering, New Jersey Institute of Technology http://srbuenaf.webs.ull.cs/potencia/hyperbolic%20location/project.html
Present and past TDOA based navigation systems usually employ an infrastructure of radio station that can simultaneously determine the position of a terminal. However, the present art also discloses less dense infrastructure to determine the position of a terminal, wherein a transmitter or receiver of such infrastructure is required to make several measurements at several locations during the fixing process.
U.S. Pat. No. 5,999,129 by Rose discloses a method and system for determining the geolocation—i.e., the latitude, longitude, and altitude—of a stationary RF signal emitter from two or more moving observer aircraft. The observers receive signals from the emitter and the system measures the phase difference between the signals. The observers then perform TOA measurements over a predetermined clock interval, and calculate the TDOA of emitter signals. Based on geometric relationships, the system creates a series of circular lines of position (LOPs) for each observer, and computes hyperbolic LOPs based on the TDOA calculations. The system determines emitter location from the intersection of the hyperbolic LOPs and the circular LOPs.
However, Rose TOA and TDOA methods for position determination of a transmitter require multiple receivers deployed at different locations.
It is therefore an object of the present invention to enable the positioning of a transmitter by a single receiver.
It is also an object of the present invention to enable the positioning of a transmitter by a single receiver, using TDOA and/or TOA measurements.
One specific need for determining the location of a transmitter using a single receiver is related to the location of radio beacons activated by people in distress, by Search and Rescue (SAR) teams.
A radio beacon (or emitter) is a device that allows tracking a ship, aircraft, an animal, or any other individual or asset. Depending on the beacon, particularly its transmitting power, but also the matching receivers or detectors, the tracking range can be as short as some meters or practically worldwide, in case of satellite served beacons.
Distress radio beacons, also known as emergency beacons: EPIRB—Emergency Position-Indicating Radio Beacon (for vessels), ELT—Emergency Locator Transmitter (for airplanes), and PLB—Personal Locator Beacon (carried by individuals), are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. In particular, EPIRBs, ELTs and PLBs are radio beacons that interface with the worldwide system of Cospas-Sarsat, the international satellite system for search and rescue (SAR). When manually activated, or automatically activated (upon immersion or collision), such beacons send out periodic distress signals, that are monitored worldwide by the system satellites, and their position is informed to SAR centers that coordinate the actual rescue.
Similar devices in the market, generally called SEND—Satellite Emergency Notification Devices (e.g. SPOT), have more or less a same functionality, yet based on commercial satellite systems.
Among other embodiments and applications, the present invention particularly addresses Cospas-Sarsat (and SEND) beacons and the following elaboration on these beacons should be regarded as a relevant example but not limiting in any way the invention.
The Objective of the Cospas-Sarsat system is to reduce, as far as possible, delays in the provision of distress alerts to SAR services, and the time required locating a distress, and providing assistance, which have a direct impact on the probability of survival of the person in distress at sea or on land.
The Strategy of Cospas-Sarsat is to implement, maintain, co-ordinate and operate a satellite system capable of detecting distress alert transmission from radiobeacons and of determining their position anywhere on the globe. By mid 2014, the Cospas-Sarsat system is comprised of a SAR segment based on LEO (Low Elevation Orbit) satellites, named LEOSAR, and another segment based on GEO (Geostationary) satellites, named GEOSAR. A SAR segment based on MEO (Medium Elevation Orbit) satellites, i.e. MEOSAR, is in development, planned to be fully operational in 2018, wherein these MEO satellites are part of the GPS, Galileo and Glonass constellations.
The future beacon, compatible with the MEOSAR, is expected to emit a direct sequence spread spectrum (DSSS) signal, enabling accurately measuring TOA and TDOA of signals emitted by those beacons, at four satellites, to provide autonomous positioning.
It is then an object of the present invention to use this DSSS communication feature to enable determining accurate TDOA and/or TOA as a way to determine the direction to a beacon and/or the location of a beacon.
A Cospas-Sarsat beacon, when activated, periodically broadcast short bursts on 406 MHz, at about 50 second intervals. These signals are typically detected by the system satellites, relayed to ground stations and then communicated to Rescue Coordination Centres (RCCs), along with a position resolution. The RCC is then responsible to conduct the actual SAR, based on the beacon position provided by the system, typically dispatching SAR teams to the distress site, by helicopters or ships or aircraft. These SAR teams, when arriving at the distress site, are not necessarily in contact with the Cospas-Sarsat system, or might not have updates on the beacon position, so need an autonomous way of locating the beacon, which might have drifted away from the last position known by the system.
Thus, though not part of the international Cospas-Sarsat standard (rather a national requirement), most 406 MHz beacons comprise an auxiliary short range transmitter operating on 121.5 MHz (civil use) or 243 MHz (military use). This auxiliary transmitter, known also as “homing transmitter” or “homer”, simply transmits at a low power, an anonymous siren tone, enabling direction finding (DF), i.e. home in on the source of the transmission.
However, Direction (or Directional) Finding of a simple RF carrier is a technology with substantial weakness. A major drawback is related to the relatively complex equipment and operational limitations and skills required at the searching side. DF typically requires directional antennas, which unless stable, cannot provide great accuracy, so quite limited in operation onboard a rolling/yawing/pitching vessel. Alternatively, more sophisticated DF antennas could be used, such as rotatable and stabilized antennas, or phased arrays, yet those are typically expensive, large, and require substantial electrical power, not quite practical for a small vessel or helicopter to carry. It also should be noted that in many cases small and private vessels are engaged in SAR, either coordinated by the RCC (which may order any vessel to assist in SAR), or upon a local accident as Man over Board (MOB).
It is then another object of the present invention to enable short range location of radio beacons by a single and simple receiver, without the need for a directional antenna, enabling an affordable, simple to install and easy to operate detecting device onboard leisure vessels.
It is still another object of the present invention to enable tracking radio beacons, such as Cospas-Sarsat EPIRB, ELT, or PLB, by SAR personnel, more efficiently than presently done.
It is yet also an object of the present invention to enable efficient tracking of a Cospas-Sarsat beacon at the beacon location, avoiding a homing transmitter, and not violating the beacon specifications.
A particular situation that the present invention addresses is the notorious Man overboard (MOB) or Person overboard (POB) accident. Man overboard is a situation in which a person has fallen from a boat or ship into the water and is in need of rescue. People may fall overboard for many reasons: they might have been struck by a part of the ship, they may lose their footing due to a slippery deck or an unexpected movement of the boat, or any number of other reasons. Falling overboard is one of the most dangerous and life-threatening things that can happen at sea. This is especially so from a large vessel that is slow to maneuver, or from a short-handed smaller boat. When single-handed and using self-steering gear it is usually fatal. Thousands of people are lost at sea every year due to MOB. Fast detection and location of such accidents is crucial, since survival time in water is short, typically under 6 hours at 10° C.
Technology can be used to assist in the retrieval of people who fall overboard. Many GPS chart plotters designed for marine use have a Man Overboard button (MOB). This button is to be pushed as soon as a Man Overboard alarm is raised, causing the plotter to record the latest known position of the person overboard. This allows the boat to be easily returned to the fallen person even if visual contact is lost.
Several manufacturers make man overboard alarms which can automatically detect a man overboard incident. The hardware consists of individual units worn by each crew member, and a base unit installed onboard. Some systems are water activated: when an individual unit comes in contact with water, it sends a signal to the base unit, which sounds the man overboard alarm. Other automatic detection systems rely on a constant radio signal being transmitted between an individual unit and the base unit; passing outside the transmission range of the individual unit and/or falling into the water causes the radio signal to degrade severely, which makes the base unit sound the man overboard alarm. Yet, present MOB alert devices, also known as Marine Survivor Locating Devices (MSLD), do not transmit data indicating their position, and as the victim drifts away, even the onboard record of last known position of the MOB becomes obsolete. At high seas and low visibility conditions, locating a MOB becomes a significant challenge.
MSLDs typically do not employ GPS, which is not efficient due to the fast and unexpected nature of MOB accidents, and the fact that almost all the victim body is typically in the water, and his/her hands (as well as the wrist worn MSLD) are typically engaged in swimming to survive in the water. Thus, most MSLDs are not based on GNSS but rather on simple low power RF emission that can be detected as long as the MSLD is onboard or up to about 100 meters away. So the burden of locating an MSLD is typically put on the searching side.
Indeed, some modern MSLDs employ AIS (Automatic Identification System) or DSC (Digital Selective Calling) transmitters, practically because many vessels are already equipped with AIS or DSC receivers, however the location data provided by AIS and DSC transmitters is based on GPS embedded receivers, and if those cannot fix the MSLD position since the GPS antenna is immersed in the water and do not allow a cold start of the GNSS algorithm, typically requiring about 30 seconds of uninterrupted satellite reception, then no valid position can be delivered by the MSLD.
It is then another object of the present invention to enable more efficient location of a Marine Survivor Locating Device, by enabling the communication of reliable and accurate location information that can be acquired and updated quickly even at extreme conditions that downgrade wireless communications, such that are typical to MOB accidents, devices that are comfortable enough to carry and affordable to purchase by every mariner.
Therefore, it is still another object of the present invention to enable MSLD detecting devices, to be installed onboard even small vessels, devices that are compact in size and low power consuming and low in cost, and easily operated even by the non professionals.
U.S. Pat. No. 7,711,375 by Liu discloses a Method and system for determining a location of a wireless transmitting device and guiding the search for the same.
Liu discloses determining the location of a wireless transmitting device using a movable detection station . . . receiving the transmitted signals . . . by said movable detection station . . . measurements at a plurality of positions of said movable detection station . . . determining at least one of the location and orientation of said movable detection station at each of said positions . . . performing estimation of the location of said wireless transmitting device . . . .
Further, Liu discloses measurement of delay of the signal propagation, from said wireless transmitting device to said movable detection station . . . measurement of difference of the signal propagation delays, from said wireless transmitting device to said movable detection station . . . wherein the difference of delays is between pairs of said positions of said movable detection station . . . .
U.S. Pat. Nos. 7,616,155 and 7,804,448 by Bull disclose Portable iterative geolocation of RF emitters.
Bull discloses a method for locating an emitter of interest (EOI) using at least one portable geolocation sensor . . . receive, time stamp and store EOI transmissions during a first period of time; moving the first portable geolocation sensor to a second location; at the second location, using the first portable geolocation sensor to receive, time stamp and store EOI transmissions during a second period of time; and computing the location of the EOI using data representative of the EOI transmissions stored during said first and second periods of time . . . .
Yet Liu and Bull are focused on location finding rather than on direction finding, and also fail to disclose transmission of periodic signals. Liu and Bull still fail to disclose providing information enabling determining the time difference between transmissions (TDOT), which could be instrumental for calculating the time difference of arrival (TDOA). Liu and Bull also fail to disclose specific formulae for determining the direction to a radio beacon (wireless transmitting device in Liu's language or RF emitter of interest—EOI, in Bull's language) while substantially moving in that direction, a scenario that is typical to beacon tracking, yet presents significant geometric dilution of precision (GDOP) for TDOA and TOA based navigation, sometimes making present art hyperbolic/trilateral navigation inaccurate to be practical.
As known in the art, Dilution of precision (DOP), or geometric dilution of precision (GDOP), or Position Dilution of Precision (PDOP) are terms used in radio navigation to specify the additional multiplicative effect of the system transmitters and receivers geometry on positional measurement precision. Basically, PDOP can be interpreted as 1/(volume of a tetrahedron, formed by the positions of the radio beacon and detecting receivers), so the best geometrical situation occurs when that volume is at a maximum and PDOP at a minimum. On the other side, when the beacon and receivers are on a same line, the volume of this tetrahedron is zero and PDOP is infinitely large.
An aspect of the DOP is reflected in the algebraic formulae of present art TDOA and TOA equations. As known in the art, the TDOA of a signal at two locations (1 and 2, forming baseline12) is associated with a hyperbola that defines a certain relationship between the x and y coordinates of the transmitter's position, such hyperbola providing a Line of Position—LOP on which the transmitter is positioned. Similarly, the TDOA of that signal detected at two other locations (3 and 4, forming baseline34), is associated with another hyperbola that defines a second relationship between the x and y coordinates of that transmitter, hence a second LOP. Geometrically, the transmitter should be placed on the intersection of these two LOPs; algebraically, these TDOA measurements provide the quadratic equations of:x2/(0.5*c*TDOA12)2−y2/[(0.5*baseline12)2−(0.5*c*TDOA12)2]=1  (1)x2/(0.5*c*TDOA34)2−y2/[(0.5*baseline34)2−(0.5*c*TDOA34)2]=1  (2)
However, if baseline12 is in line with the transmitter, then baseline12=c*TDOA12, thus at equation (1) a zero appears in the denominator (under y2) and the hyperbola is undefined on that line. In case that baseline34 is in line with the transmitter, then at equation (2) a zero appears in the denominator.
The PDOP issue is also relevant to TOA measurements, i.e. when the locus of points meeting a radio navigation measurement is a circle (in 2D, and sphere in 3D). Considering two receivers forming a baseline which is in line with a radio beacon to be positioned, then TOA measurements made by these receivers define two circles on which the radio beacon is positioned, however these circles do not cross each other but have a single tangential point, and in the presence of even low noise or slight measurement error, might have no common point at all. The algebraic representation of such degenerated case is of two dependent (or inconsistent) quadratic navigation equations that do not provide two pairs of solutions for the beacon coordinates (x, y) as in the general case, but either an infinite number of solutions (circles overlap) or no solution at all (circles do not intersect).
It should be noted that for simplicity, the present analysis is done in 2D, yet the mathematical representation of 3D hyperbolic (i.e. hyperboloid) navigation based on TDOA is well known in the art (and so is the case with TOA navigation providing 2D circles and 3D spheres), as described in the two papers indicated above, so persons skilled in the art can easily migrate from 2D equations to 3D equations.
So it is a further object of the present invention to enable determining the location of a radio beacon employing a single receiver in TDOA/TOA measurements, while moving in the direction of the beacon. Such a need is typically important when it is required to physically arrive at the beacon using said receiver, as in SAR operations, where it is particularly paramount to arrive quickly at the beacon.
Essentially, it is another object of the present invention to enable determining the direction to a radio beacon employing a single receiver in TDOA/TOA measurements, while moving in that direction.
It is an additional object of the present invention to enable small and low power (operated on small batteries) and low cost devices attached to people, animals or any other asset or object, to send data that may assist in the location thereof, even where or when no GNSS signals are available.
Other objects and advantages of the invention will become apparent as the description proceeds.